Double patterning is a technology developed for lithography to enhance the feature density. Typically, for forming features of integrated circuits on wafers, lithography technology is used, which involves applying a photo resist, and defining patterns on the photo resist. The patterns in the patterned photo resist are first defined in a lithography mask, and are defined either by the transparent portions or by the opaque portions in the lithography mask. The patterns in the patterned photo resist are then transferred to the underlying features.
With the increasing down-scaling of integrated circuits, optical proximity effect posts an increasingly greater problem. When two separate features are too close to each other, the optical proximity effect may cause the features to short to each other. To solve such a problem, double patterning technology is introduced. The features closely located are separated to two masks, with both masks used to expose the same photo resist. In each of the masks, the distances between features are increased over the distances between features in the otherwise single mask, and hence the optical proximity effect is reduced, or substantially eliminated.
When the double patterning technology is used in the formation of metal layers, which are used to route connections between integrated circuits, problems may arise, and design rules may be violated. To solve this problem, design rules may be relaxed by increasing the pitches of the metal lines. However, this defeats the purpose of using the double patterning technology, which is for laying out features tightly in order to save the chip area. An additional method is to perform additional iterations of routing to fix the hot spots that may be generated in preceding routing iterations. However, any fix to the routing may have ripple effects, and may require the re-routing in the entire chip. New methods are thus needed to improve the efficiency in the metal routing.